Slow positron beams have been commonly used in positron microscopes, for research in physical properties and for crystal defect evaluation of the surfaces or interfaces of semiconductors and metallic materials, and recently have become useful more and more. At present, slow positron beams are generated by emitting from positron emitters (radioisotopes), or by ejecting positrons that are generated through pair creation with a braking radiation into a moderator to be slowed down the positrons. A positron emitter is often prepared by irradiating a solid target (e.g., aluminum or boron nitride) with a beam of charged particles (e.g., protons) accelerated with a cyclotron or the like; thus a positron emitter can be generated in the solid target. A braking radiation is usually generated by irradiating a heavy metal target with an electron beam accelerated with a linear accelerator or the like.
Upon the utilization of positron beams, a strong point source for a positron emitter is required. Various approaches have been proposed for increasing the intensity of positron beams, such as the improvement in moderator efficiency and the use of a stronger positron source. As a moderator, one formed of a tungsten foil which is annealed at 2000.degree. C. is currently used. However, such moderator cannot achieve an efficiency of the order of 10.sup.-4 or more. Although many efforts are being made to improve moderators, drastic and practical improvements could hardly be expected. On the other hand, for preparing a strong positron source, the use of a large-scale and expensive device is needed.
In the preparation process for a strong positron source using a solid target, there is a serious problem that heat generated during the passage of a large electric current should be removed. The process also has another problem as follows. A solid target is placed nearby a moderator for the purpose of causing to emit positrons from a positron emitter generated in the target and increasing the incident efficiency of positrons generated through pair creation with a braking radiation into the moderator. When such solid target is irradiated with an electron beam or an ion beam, the moderator sustains a radiation damage or is radioactivated by a secondary radiation other than the positrons. In order to overcome this problem, it is proposed an approach for avoiding the influence of the secondary radiation during the irradiation of the target, which comprises: irradiating a solid target at a place a distance away from a moderator thereby generating a positron emitter; transferring the irradiated solid target to the place where the moderator is placed; and ejecting a beam of positrons emitted from the positron emitter in the solid target into the moderator. However, such approach is not practical. This is because the use of a solid target usually needs a cooling device for removing heat generated as a result of the irradiation and, therefore, if a solid target is to be transferred, the system as a whole will inevitably become a large scale due to the integration of the cooling device. In the process utilizing a braking radiation generated with an electron beam, it is impossible in principle to separate a heavy metal target and a moderator. Moreover, in this process, it is necessary to automate the supply of a positron source to a positron beam-generating unit for the purpose of avoiding the harmful irradiation exposure of operators.